In early days, in rich family homes, bathtubs were placed in the middle of the room. Shower curtains ran around the entire tub on curtain rods that hanged from the ceiling.
In this modern time, bathtubs are placed in one small room and it is surrounded by 3 walls. This leaves only one side of the tub to be curtained. Curtains are hanged on straight curtain rods that are mounted between the opposite walls either by the rod itself that is self supporting by pushing itself hard against the wall or using socket mounts on both ends that are screwed to the wall.
Straight curtain rods are mounted along the outside end of the bath tub. The curtain ends, that are very close to the outside of the tub do not stop water from spilling onto the floor when one is taking shower. It would have been better if these curtain ends are deeper inside the tub, to the middle of the tub as an example. The curtain ends will act like shield and the problem of water spilling onto the floor will be greatly reduced.
The curved shower curtain rod is straight in the middle but curved at both ends. The middle of the curtain rod is still along the outside edge of the tub. The ends curve towards inside to the middle of the tub.